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among rhizobial strains (Young and Chao 1983), Wu (1958) selected a number of pure rhizobial
strains from lupin, alfalfa, peanut, crotalaria, and soybean and conducted a wide range of field
experiments to select the effective inoculants. Yield was significantly increased when lupin,
alfalfa, peanut, and soybean were inoculated with selected rhizobial strains, compared to those
with non-inoculated plants.
After the 1980s, fast- and slow-growing soybean rhizobial strains were isolated and selected
from Taiwan soils for inoculation (Young et al., 1982; Young and Chao, 1983), and several
effective isolates were deposited in the Culture Collection and Research Center (CCRC) of the
Food Industry Research and Development Institute (CCRC, 1991).
Field experiments have been conducted to determine the effects of single and mixed inoculations
with rhizobium and Arbuscular-Mycorrhiza (AM) in six different tropical Taiwan soils
(Young et al., 1988b). The results indicated that inoculation with rhizobial strains alone increased
N2 fixation and soybean yield in three out of six fields. Inoculations with rhizobial
strains singly, or in combination with AM, without any N2 fertilizer application, significantly
increased soybean yield, from 5% to 134%, in the field experiments. The results from other
experimental sites also showed that a mixed inoculum of rhizobium and AM can be an efficient
biological fertilizer that maximizes soybean yields. The combined effect of the mixed inoculum
was a striking finding in the field of biofertilization. The AM might have provided the essential
P for the growth of soybean plants.
P-solubilizing Microbial Inoculants
P-solubilizing bacteria have been isolated from various tropical soils of Taiwan. Aliquots of
soil diluted with sterile water (1:10 soil/water) were plated on calcium phosphate medium
(modified from Subba Rao, 1982) for the isolation of P-solubilizing bacteria.
The basic research on P-solubilizing biofertilizers was successfully established during the
1990s (Young, 1990; Chang and Young, 1992a&b; Young et al., 1998a&b; Chang and Young,
1999, Young et al., 2000; Liou and Young, 2002; Young et al., 2003a,b&c). Crop plants such as
peanut, various horticultural plants, and vegetables were successfully inoculated with PSBs to
obtain higher yields. Several field experiments concluded that P-solubilizing bacteria not only
improved the growth and quality of crops but also drastically reduced (one-third to one-half) the
usage of chemical or organic fertilizers.
A-Mycorrhizal Inoculants
The major VAM fungi used as inoculants are Glomus spp. isolated from tropical soils of
Taiwan (Young, 1986). Chlamydospores are borne terminally on single undifferentiated hyphae
in soil. The mature spores were separated from the attached hyphae by a septum. The AM fungal
inoculant was placed in pots containing sterilized mineral attapulgite [(Mg.Al)5Si8O22(OH)4.
4H2O] with Zea mays as the host plant. The VAM fungal inoculant used in pot experiments contained
approx. 50 spores/g soil together with infected roots (Young et al., 1988b).
Young et al. (1986) used two species of AM in a pot experiment to observe the effects of
inoculation of AM fungi on the yield and mineral P utilization in soybean. The results showed
that the AM fungi inoculation increased soybean yields over the uninoculated treatments, but
results depended on the soil type. Moreover, the P uptake by soybean was significantly improved
in the inoculated treatments. In a similar experiment, rhizosphere soil was used to assess
the difference in P uptake by soybean plants. Soybean in non-inoculated treatments took up
minimum Al-P from acidic soils, less Ca-P from calcareous soil, but failed to absorb Fe-P from
any soil type. Inoculation with either of the two mycorrhizal fungi improved the uptake of Al-P
by soybean in acidic soils and also increased the uptake of Ca-P in calcareous soils, and a significant
amount of Fe-P uptake was evidenced. These results suggested that AM can enhance
uptake of fixed soil P. The efficiency rate and utilization of various forms of mineral P by
mycorrhizal plants depends on the species of mycorrhizal fungi inoculated and on the soil type.
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